This invention relates generally to cellular telephone communications, and more particularly to a system and method for mobile stations to use the timing information derived from the perch channel of an asynchronously transmitting base station to demodulate a traffic channel message.
Spread spectrum communication techniques allow communicating users to operate in noisy radio frequency (RF) spectrums, and are especially effective against narrow-band interferers. Spread spectrum communications can be effected at relatively low power spectral densities, and multiple users can share the same frequency spectrum. Further, receivers can be designed to protect against multipath. These system characteristics encouraged early development of the technology by the military.
Common forms of spread spectrum systems include chirp, frequency hopping, and direct sequence or pseudonoise (PN). The chirp system transmits an impulse signal in the time domain that is spread monotonically in the frequency domain. A receiver converts the spread frequency signal back into an impulse signal. These frequency-spread impulse signals have applications in radar, for the pulse position modulation of information, or both, such as the R.sup.3 transponder developed by General Dynamics, Electronics Division in the 1970s. Frequency hopping systems communicate by synchronizing users to simultaneously change the communication frequency.
Direct Sequence systems spread a digital stream of information, typically in a quadriphase modulation format, with a PN code generator, to phase shift key modulate a carrier signal. The pseudonoise sequence of the PN code generator is periodic, and the spread signal can be despread in a receiver with a matching PN code. Direct Sequence systems have excellent immunity to noise. The PN codes used typically permit a large number of users to share the spectrum, with a minimum of correlation between the user's PN codes. However, Direct Sequence system require large RF bandwidths and long acquisition times.
The IS-95 standard defines key features of the so-called second generation code division multiple access (CDMA) communication system, a type of Direct Sequence spread spectrum modulation. To help solve the problem of long acquisition time, the IS-95 signal uses a pilot channel. Each base station transmits a pilot channel message spread with PN codes known to all the mobile stations. The PN code is made up a series of phase shifted binary symbols called chips. The PN period is 32,768 chips and the PN chip rate is 1.2288 Megahertz (Mhz). The digital stream of information that is spread by the PN code is also known to the mobile stations. Because there is no ambiguity in the demodulated message, the timing characteristics of the PN code, down to the chip phase, as well as the QPSK modulation phase are known to the mobile station receiver.
The IS-95 system communicates information from the base station to the mobile stations through a series of traffic channels. These traffic channels are transmit and receive information, i.e. digitized audio signals, spread with a traffic channel PN code, unique to each mobile station. Using this precise timing and phase information derived from the pilot channel, the mobile station is able to acquire a setup channel, and eventually, the overall System Time. With this System Time, the mobile station is able to differentiate between base stations and synchronize the demodulation circuitry with sufficient accuracy to recover the received traffic channel message.
A third generation, wideband CDMA (W-CDMA) system is in development as described in "Wideband-CDMA Radio Control Techniques for Third Generation Mobile Communication Systems", written by one et al., IEEE 47.sup.th Vehicular Technology Conference Proceedings, May 1997, that may have global applications. Instead of a pilot channel, the W-CDMA system has a broadcast, or perch channel. Each timeslot, or slot of the broadcast channel consists of a series of time multiplexed symbols. A long code masked, or special timing symbol segment uses just a short code to spread one symbol of known information. This segment allows a mobile station to acquire system timing information immediately after turn-on. The pilot, or reference symbols are similar to the IS-95 pilot channel. In one proposal, 4 reference symbols, with each symbol being 2 bits, are spread with a long code and a short code. The reference symbol information and the short code are known by the mobile stations. The long code is unique to each base station, so that timing information is refined, once the long code is known (the base station is identified). Therefore, according to some proposals, 5 symbols in the slot would be dedicated to the mobile station acquiring timing information. Further, both the long and short codes spread 5 symbols of data during each slot. Since information is not predetermined for the data symbols, precise timing information cannot be accurately recovered, as with the other two kinds of (timing) symbols. Other combinations of reference, special timing, and data symbols are also possible.
The W-CDMA system also includes several traffic channels to communicate information, such as a digitized voice or data. The traffic channel predominately includes information, but may also include a reference symbol segment. For example, at a data rate of 32 kilosymbols per second (ksps), a slot could include 4 pilot symbols and 16 information symbols. Precise timing information can be derived during the reference symbols segment of the traffic channel message, but not during the information segments.
The W-CDMA, or any spread spectrum system, operates best by minimizing the transmitted power of the users. Lower spectral power densities permit additional users to be added to the system, or an increase in the signal to noise ratio of received messages. Each mobile station is likely to receive more than one traffic channel from a base station, with each traffic channel being unique to a mobile station. That is, each base station is capable of transmitting hundreds of different traffic channels, the exact number is dependent on the traffic channel data rates. However, each base station transmits only a few, perhaps only one, broadcast channels that are used by all the receiving mobile stations. It is advantageous for the system that the base stations transmit the shared broadcast channels at a higher power level than the mobile station specific traffic channels. For this reason, the broadcast channel power is maintained at a relatively high level, while the traffic channel levels are continually monitored and adjusted to keep the transmitted power levels only as large as necessary to reasonably enable communication between the base station and the mobile.
Unlike the IS-95 system, the W-CDMA system does not use a master System Time to synchronize the base station transmissions. Each mobile station must independently acquire sufficient timing information regarding each base station to recover messages from that base station. The mobile station must simultaneously maintain timing information for multiple non-synchronously transmitting base stations.
Gilhousen, et al., U.S. Pat. No. 5,109,390, disclose a spread spectrum receiver capable of differentiating multiple pilot signals and selecting the signal of greatest strength. The transmitting base stations are synchronized to operate from a master clock. Receiving mobile stations can maintain timing accuracy sufficient to demodulate received messages from all base stations by monitoring the pilot channel of any single base station. However, Gilhousen et al. do not disclose a method of conveniently receiving communications from asynchronously transmitting base stations.
Ling, U.S. Pat. No. 5,329,547 discloses a method of inserting reference symbols into a stream of spread spectrum data symbols. The reference symbols help generate a channel estimate. That is, the insertion of predetermined data, or reference symbols into the data stream helps eliminate phase ambiguity in demodulating unknown data symbols. However, Ling does not disclose a method of synchronizing the timing between a plurality of received channels.
It would be advantageous if a W-CDMA receiver were developed to acquire the channels of a first asynchronously transmitting base station independent of the transmissions of other base stations. It would also be advantageous if the receiver could direct a plurality of base stations to synchronize transmissions to take advantage of the diversity provided by receiving from several base stations.
It would be advantageous if the base station transmitted broadcast, or perch channel could be used by a mobile station to maintain timing for all channels received from that base station.
It would be advantageous if the amount of channel information present in the structure of a receiver multi-channel CDMA waveform was maximally utilized.
It would be advantageous if the broadcast channel in a W-CDMA system, generally having more transmitted power than the traffic channels and a greater number of reference symbols, could be used by a mobile station to demodulate the traffic channel.
It would be advantageous if the channel estimates derived from the broadcast channel could be applied to all the received channels of the same transmission path. In this manner, the channel estimate need be performed only once.
Accordingly, a method of receiving communications in a CDMA communication system, including a plurality of base stations asynchronously transmitting information to a plurality of mobile stations, is provided. The communications from a base station to a mobile station are formatted in a plurality of channels. Due to multipath, these communications are propagated along at least one transmission path, with a corresponding path delay. Each of these communications can be considered a family of related channels, so that families of channels propagate along the same transmission path. A method for each mobile station to receive base station communications comprises the steps of:
a) for each base station from which a communication is received, identifying at least one transmission path between a base station and the mobile station. Typically a mobile station is able to identify a base station communication along several transmission paths; and PA1 b) in response to the transmission paths identified in Step a), despreading at least one received communication. That is, in an asynchronous system of transmissions, the mobile station is able to recover the data symbols in the communication in response to timing information recovered from locking the receiver onto any one of the multipathed base station transmissions. PA1 a.sub.1) in response to special timing symbol despread in Step a), calculating channel timing information for each transmission path detected in Step a); and PA1 c) in response to the broadcast channel reference symbols despread in Step b), demodulating the broadcast channel reference symbols to provide transmission path weights and phase shift information; PA1 d) in response to the weights and phase shifts provided is during the demodulation of the broadcast channel reference symbols, estimating weights and phase shifts to apply to data symbols; and PA1 e) in response to estimations made in Step d), demodulating the broadcast and traffic channel data symbols.
Each base station transmission includes a broadcast channel message with a plurality of predetermined time multiplexed symbols, including a predetermined special timing symbol known to each mobile station. Step a) includes, for each transmission path identified in Step a), despreading the special timing symbol, whereby broadcast channel multiplex timing information is derived. Further steps include:
in which Step b) includes despreading received communications in response to the channel timing information calculated in Step a.sub.1).
The broadcast channel message includes time multiplexed data symbols, and predetermined time multiplexed reference symbols known to the mobile station. Step b) includes despreading the broadcast channel data and reference symbols. Initially, the channel timing is found by despreading the special timing signal in Step a.sub.1). The timing is improved by despreading the reference symbols in Step b).
Each base station transmits at least one traffic channel message, unique to each mobile station. The traffic channel has a plurality of time multiplexed data symbols. Step b) includes despreading the traffic channel data symbols. Since the reference and data symbols for both the traffic and broadcast channels are modulated before transmission, the method of the present invention includes the further steps, following Step b), of:
Each traffic channel message includes predetermined time multiplexed reference symbols known to each mobile station, which are modulated before transmission. In some aspects of the invention, Step b) includes despreading the traffic channel reference symbols. Then, the traffic channel reference symbols are demodulated to provide transmission path weights and phase shift information. In addition, the weights and phase shifts provided from the demodulation of the traffic channel reference symbols are used to estimate weights and phase shifts to apply to the demodulation of the traffic channel data symbols.
Ultimately, the broadcast channel data symbols demodulated for each transmission path are combined in a RAKE receiver to improve the signal to noise ratio of a received message. This process occurs by combining the received transmissions of each base station.
A mobile station receiver to accept base station communications is provided. The receiver comprises at least a first filter matched to despread the broadcast channel special timing symbol. The first matched filter accepts the broadcast channel special timing symbol received for each transmission path from a communicating base station, and provides the despread special timing symbol for each transmission path.
The receiver also comprises a timing and code management circuit connected to the first matched filter output to accept despread special timing symbols for each transmission path. The timing and code management circuit provides the despread broadcast channel special timing symbol for each transmission path, and a second output provides the broadcast channel multiplex timing information and base station identification for each transmission path.
The receiver comprises a traffic channel RAKE receiver having a plurality of fingers. Each finger is operatively connected to the timing and code management circuit second output. Each finger uses the broadcast channel multiplex timing and base station identification information provided by the timing and code management circuit to despread the traffic channel data symbols for each transmission path. The timing and coding needed to despread the traffic channel results from despreading the broadcast channel.
Each base station is assigned a unique long code, and the base stations transmit broadcast channel reference symbols spread with their long code. The mobile station's receiver further comprises a searcher unit to accept the broadcast channel reference symbols for each transmission path. A second searcher input, connected to the timing and code management circuit, accepts the despread broadcast channel special timing symbol for each transmission path. The searcher unit identifies the long code for the broadcast channel received on each transmission path, and provides the long code of the broadcast channel for each transmission path. The timing and code management circuit, connected to the searcher, accepts the broadcast channel long code for each transmission path. The timing and code management circuit second output provides broadcast channel long codes, as well as despread special timing symbols, for each transmission path.
The mobile station receiver further includes a broadcast channel RAKE receiver with a plurality of fingers, with each finger assigned to a received message transmission path. The broadcast channel RAKE receiver comprises a delay locked loop (DLL) to provide a clock at a first chip rate and a long code despread signal, which is the product of the long code times the channel messages of a transmission path. The DLL also despreads the broadcast channel message. A broadcast channel estimation and weighting circuit demodulates the broadcast channel reference symbols to determine the assigned transmission path weights and phase shifts, and to estimate weights and phase shifts to apply during the demodulation of broadcast and traffic channel information symbols. A first summing circuit combines the demodulated broadcast channel data symbols.
The receiver comprises a traffic channel RAKE receiver with fingers corresponding to the broadcast channel fingers having the same assigned transmission path. Each traffic channel RAKE receiver finger despreads and demodulates the traffic channel data symbols. A traffic channel estimation and weighting circuit accepts the despread traffic channel data symbols, and is connected to the broadcast channel estimation and weighting circuit to accept the estimated weights and phase shifts to aid in the demodulation of the traffic channel data symbols. In some aspects of the invention, the traffic channel estimation and weighting circuit also demodulates the traffic channel reference symbols to determine the assigned transmission path weights and phase shifts, and estimates the weights and phase shifts for application in the demodulation of traffic channel data symbols. Several second summing circuits, each second summing circuit being assigned the various transmission paths of one base station, combine the outputs of several traffic channel estimation and weighting circuits. A third summing circuit combines the results from each base station (each second summing circuit) to improve the quality of the received traffic channel message.